Comparison of Absorbed Dose in Thyroid and Lens as Organs at Risk Between in vivo Dosimetry and 3-D Treatment Planning Calculation in Head and Neck Radiotherapy by Linac Beam

Introduction: Critical organs and structures may receive significant amounts of irradiation even if they are not the target of radiotherapy or located outside the treatment field. Although the sensitive thyroid gland and lens are not directly the targets of treatment, they can be affected by irradiation during the treatment of tumours in head and neck region. The purpose of this project is to compare measured dose by TLD and calculated dose in 3Dtreatment planning system (3DTPS) in thyroid and lens as critical organs during radiation therapy for patients with head and neck cancers. It can be improved treatment planning process to reduce absorbed doses of critical organs and side effects in radiotherapy. Beside knowing the absorbed dose of critical organs can be estimate secondary cancer in these organs. Materials and methods: In this study the absorbed dose of thyroid and lens in anthropomorphic phantom and also 20 patients suffering from tumors in head and neck region under treatment by linac (varian 2100), measured by thermoluminesencent dosimeter chips (TLD-100). The results compared to calculated doses in 3DTPS called CorePlan in ETAR algorithm and analyzed by SPSS 19 software in paired T-Test. The secondary cancer risk was calculated using data provided in National Council on Radiation Protection & Measurements (NCRP) 116. Results: According to the measurements performed on phantom the results of average absorbed dose in nasopharynx field organs were as follows in order: thyroid 190.94 ± 3.8 cGy/F, lens 10.3 ± 0.8 cGy/F, by TLD, respectively, and 195.2 ± 5.3cGy/F, 10.7 ± 0.2 cGy/F calculated by 3DTPS. In larynx field the results were: thyroid 181.1 ± 2.6 cGy/F, lens 1.1cGy/F, measured by TLD, respectively, and 186.1 ± 33.7 cGy/F, 0 cGy/F, calculated by 3DTPS. According to the treatment planning performed on the patients the results of average absorbed dose were in thyroid 4253.16 ± 1013 cGy and in lens 253.58 ± 84.2 cGy. The measurement of average absorbed dose by TLD in lens was 238.67 ± 83.4 cGy. Results revealed the fatal cancer risk for thyroid was about 3.4%. Conclusion: The results showed that there are no significant differences between 3DTPS calculation and TLD measurement in thyroid and lens organs. It can be used of 3DTPS to estimate the absorbed dose of these organs in head and neck cancers. This study showed that the thyroid received a dose near to the TD5/5 in reported by Emami et al, which encourages recommendation of thyroid function follow up after radiotherapy in head and neck cancers.